Modern radiation detectors are based on high-voltage semiconductor devices. The semiconductor substrates are commonly silicon, germanium, III-VI compound semiconductors, or II-VI compound semiconductors. Silicon is the most commonly used semiconductor. The basic operation is similar to a photo detector. The radiation hits the detector and generates charge carriers which are then collected on either a top or bottom surface of the device. In order to achieve good charge collection from the full depth of the detector, voltages up to several thousands volts are applied. For imaging applications, the radiation detectors need to be segmented. Pixel or strip detectors are the most common examples. Detectors that have strips on the front and back side are called double-sided strip detectors (DSSD). DSSDs and pixel arrays allow for full 2D positioning, depending on pixel and strip dimension, where position accuracy down to ˜10-20 μm is possible.
Radiation detectors can be made of n-type or p-type silicon. Modern high energy physics experiments uses p-type silicon (Si) because p-type Si is more radiation hard in comparison to n-type Si. Strip detectors on p-type silicon require an “inter-strip isolation” to isolate them from each other. Between the strips a silicon oxide layer, such as Silicon Dioxide (SiO2), is used as a dielectric. Since the Si—SiO2 interface has a fixed positive charge, electrons from the bulk silicon can accumulate at the silicon surface and short neighboring strips to each other. In order to provide the necessary inter-strip isolation, “p-stops” are used. P-stops are junctions surrounding n-type strips, which avoid electron accumulation underneath the silicon oxide dielectrics. An alternative approach is the use of a “p-spray”, where a blanket ion implantation is used to achieve the required inter-strip isolation. P-stops and p-sprays are implanted junctions, which require additional fabrication steps.
For n-type silicon detectors the situation is different. P-stops are needed for the n-on-n strips. N-on-n strips are most commonly used for double-sided-strip detectors.
The simplest type of semiconductor radiation detector involves observing the change in conductivity in a semiconductor due to the creation of additional charge carriers by an incident electromagnetic field or ionization created by a charged particle. These devices are commonly known as photoconductors. Semiconductor junction photodiodes can exhibit dramatically improved performance over simple photoconductor detectors. Photodiodes may be designed with improved response times, greater sensitivity, decreased thermal sensitivity, linearity over 9-10 orders of magnitude, large internal amplification, and may also be used to generate power at levels comparable to the power which can be generated by solar cells. The most common form of photodiode used is the silicon positive-intrinsic-negative (PIN) diode in which a thick layer of an intrinsic semiconductor material is grown between the p and n layers of the junction. The same structure can be made in germanium or any compound semiconductor. PIN diodes are normally fabricated using n-type silicon substrates with a bulk resistivity >1,000 Ωcm.
Referring to FIG. 1, positioning is accomplished by segmenting radiation detectors into radiation detector segment strips 104 or pixels on a silicon substrate or wafer, such as the p-substrate 114. One of the major technological challenges in the fabrication of n-in-p (n-type segment strips 104 in p-type Si) microstrip silicon detectors is to achieve a good isolation (i.e., separation) between the strips 104 (i.e., having good inter-strip isolation) at the n-side, while ensuring the satisfactory electrical performance of the devices during their life span. The inter-strip isolation is necessary, because the positive charge in the SiO2 induces the creation of an electron accumulation layer 102 at the (oxide 106-silicon 108) interface 130, increasing the inter-strip capacitance and eventually shorting the strips 104 (such as strip 1, also identified as strip 104 and strip 2, also identified as strip 104) together, see FIG. 1.
FIG. 2 shows schematically the charges at the silicon/silicon oxide interface 130, where there is a non-stoichiometric SiO2 layer, which has a positive charge labeled Qf—fixed 202, according to S. M. Sze, Physics of Semiconductor Devices, Wiley-Interscience: 2nd edition (September 1981). This positive charge attracts electrons.
FIG. 3 shows a finite element simulation (FEM) of the electron concentration distribution for an oxidized n-type silicon substrate (2,000 Ωcm), underneath a silicon oxide layer (substrate high resistivity n-type Si). The positive charge (1011 cm−2) leads to an electron accumulation at the silicon/silicon oxide interface 130 Qt.
FIG. 4A, FIG. 4B, and FIG. 4C illustrate three different structures for inter-strip isolation. The p-spray 402 isolation technique consists of a medium dose p-implant which is applied to the entire n-side and is overcompensated by high dose n+ pixel implants. The p-stop 404 is an implanted layer underneath the oxide. Since there will be an electron accumulation underneath the oxide, a high-field region develops at the edge of the p-stop 404. High-field regions are problematic because they can lead to catastrophic breakdowns during high voltage operation. The third type of inter-strip isolation is a combination of (p-spray/and p-stop) 406. This technique leads to a step in the effective p-spray 402 dose along the gap between two n+-implants. In the middle of the gap, the normal p-spray 402 dose is reached. This guarantees the inter-pixel and/or inter-strip isolation. Near the edges, the p-spray dose is lower in order to minimize the electric field strength in these regions and to thereby improve breakdown performance, (i.e., see FIG. 4A, FIG. 4B, and FIG. 4C, which illustrate the p-spray 402, p-stop 404, and (p-stray/p-stop) 406, respectively. The high-field region depends on the design of the inter-strip isolation).
The inter-strip shorting due to electron accumulation is a problem for any segmented p-type and double-sided n-type detectors. The standard approach for inter-strip or interpixel isolation is the use of an implanted and annealed p-type layer.
P-stop or p-spray implants need to be annealed for implant action. This annealing step leads to an additional heat load for the sensor by a high temperature process step. Ideally, minimization of high temperature processes is more desirable in detector fabrication. Furthermore, any high temperature step increases the risk of introducing contamination.
Therefore, the need exists for a fabrication method of producing radiation detectors at reduced costs (preferably at a low temperature, <500° C.), using fewer fabrication steps than conventional fabrication methods (thus precluding the need for p-stop fabrication steps), while achieving segment isolation.
Further, the need exists for the replacement of silicon oxide with a dielectric which circumvents the problem of inter-strip shorting based on electron accumulation, due to the negative interface charge of the dielectric. Furthermore, atomic layer deposition is a low temperature process, reducing the overall thermal load.